1. Field of the Invention
This invention relates to position or velocity sensors.
2. Description of the Related Art
Position and velocity sensors are often based on the effect of external magnetic fields, e.g. according to the magneto-inductive or the magnetostrictive functional principle.
In such sensors, the current position of the component to be monitored is determined by detecting a position indicator mounted to the component with respect to its position relative to the remainder of the sensor, thus the sensor unit.
Depending on the functional principle, the position indicator can be a metal part or a magnet which can either be disposed and moved on one side next to the sensor unit or can enclose it in an annular manner as a bushing. Hereinafter, a magnetostrictive sensor is referred to in an exemplary manner in order to designate the components in a simplified manner, without restricting the invention, in which the sensor unit is designated as wave conductor unit and the position indicator is designated as magnet or ring magnet.
Thus, a typical application for this is a hydraulic cylinder or pneumatic cylinder in order to determine the position of the piston in the cylinder.
The wave conductor unit is thus mounted to the head of the cylinder in a solid manner and protrudes into a center bore in the interior of the piston. The position indicting ring magnet is thus mounted in the forward, expanded end of the center bore in the piston rod.
Though the interior diameter of the annular magnet is selected larger than the exterior diameter of the wave conductor unit, both often slide on each other during their relative motion, since the wave conductor unit is fixed only on one side, and can be laterally deflected in a substantial manner due to lengths of up to 6 m, this can occur through oscillations or jolts during the operation, or also simply due to its own weight, in case of a non-vertical, but slanted or horizontal installation.
In case of a friction induced wear on the ring magnets, which can lead to the destruction of the magnet and thus to the non-functionality of the entire sensor and necessitating a replacement of the magnet, the ensuing costs through shutdown times of the respective machinery are typically higher than the cost of the sensors itself.
Often a canting of the magnet, relative to the wave conductor unit, and an associated kinking and destruction of the wave conductor unit occurs which is also caused by a previous undue wear at the interior circumference of the magnet or by an insufficient axial guidance length.
Also, the bushings made from aluminum or brass, which were used so far for receiving the magnet, have only partially solved these problems, since their friction coefficients were not optimum, neither were their guidance length.
In addition, there is the possibility that the wear properties, the occurring abrasion material, the canting of the annular magnet, and the kinking of the wave conductor unit are facilitated by the locally occurring pressure conditions within the cylinder which occur though the fact that the magnet acts as a throttle location for the hydraulic medium.
Thus, for functionality reasons, hydraulic fluid does not necessarily have to penetrate into the central bore for the wave conductor unit in the piston rod beyond the ring magnet, however, the ring magnet never seals this space reliably, since it would have to withstand the high working pressure in the piston.
On the other hand, the radial clearance is too small for the ring magnet not having to be considered as a throttle body for the fluid flow into this space.